1. Field of the Invention
The present invention relates to protection switching for working transmission systems using a spare transmission system in a hot-standby mode.
2. Description of the Related Art
It is known that a conventional (n+1) protection switching system includes a switching module for establishing "n" two-way connections between corresponding "n" two-way channels and corresponding "n" working transmission systems when each of the working transmission systems is operating properly. When one of the working transmission systems fails, the switching module is controlled to clear the connection between the corresponding channel and the failed working transmission system and a new connection is established between that channel and a spare transmission system. The switching module is an integrated assembly of relays and the like which cannot be individually taken out of the module for repair. If a failure occurs in one of these relays, the whole unit of the module is discarded for replacement with a new switching module. However, this is undesirable from the standpoints of economy and protection of environment. Another disadvantage of the prior art protection switching module is that when a protection switching occurs in the switching module in response to the occurrence of a failure in one of the working transmission systems, relay contacts are directly interposed between the corresponding channel and the spare transmission system, causing a signal loss variation and an impedance variation which result in a lowering of transmission efficiency.